Inspection apparatus for detecting defects in photomasks and dies

ABSTRACT

A defect inspecting apparatus includes a reference image generator configured to generate a first reference image and a second reference image from design layout data. An image inspector is configured to obtain a first inspection image of a first inspection region of a photomask and a second inspection image of a second inspection region of the photomask. An operation processor is configured to extract a first coordinate offset by comparing the first inspection image with the first reference image and to extract a second coordinate offset by comparing the second inspection image with the second reference image.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of co-pending U.S. patent applicationSer. No. 16/013,417, filed on Jun. 20, 2018, which claims priority under35 U.S.C. § 119 to Korean Patent Application No. 10-2017-0146122 filedon Nov. 3, 2017 in the Korean Intellectual Property Office, thedisclosures of which are herein incorporated by reference in theirentirety.

TECHNICAL FIELD

The present inventive concept relates to defect detection and, morespecifically, to an inspection apparatus for detecting defects inphotomasks and dies.

DISCUSSION OF RELATED ART

Design rule checking is a process by which automated hardware is used toverify that the physical layout of a photomask or integrated circuitsatisfies various constrains, which are referred to as design rules. Asmicrochips become more highly integrated and the size of circuitfeatures thereon continues to shrink, the design rules specify evertighter constraints. Additionally, as microchips become more highlyintegrated, the size of the photomask patterns used to patternsemiconductor devices have also been reduced. As such, as the size ofthe mask pattern is reduced, the time required for performing designrule checking has increased.

SUMMARY

A defect inspecting apparatus includes a reference image generatorconfigured to generate a first reference image and a second referenceimage from design layout data. An image inspector is configured toobtain a first inspection image of a first inspection region of aphotomask and a second inspection image of a second inspection region ofthe photomask. An operation processor is configured to extract a firstcoordinate offset by comparing the first inspection image with the firstreference image and to extract a second coordinate offset by comparingthe second inspection image with the second reference image.

A method of inspecting a defect in an inspection object includesgenerating a first reference image and a second reference image fromdesign layout data pertaining to the inspection object. A firstinspection image and a second inspection image of the inspection objectare obtained. A first coordinate offset is extracted by comparing thefirst inspection image with the first reference image. A secondcoordinate offset is extracted by comparing the second inspection imagewith the second reference image. The first inspection image is comparedwith the second inspection image in a die-to-die manner using the firstcoordinate offset and the second coordinate offset. It is determinedwhether a defect is present in the second inspection image based on thecomparison of the first and second inspection images.

A method of inspecting a photomask for defects includes generating afirst reference image and a second reference image from design layoutdata of the photomask. A first inspection image of a first inspectionregion of the photomask is obtained by directing light onto thephotomask. A second inspection image of a second inspection region ofthe photomask is obtained by directing light onto the photomask. A firstcoordinate offset I extracted by comparing the first inspection imagewith the first reference image. A second coordinate offset is extractedby comparing the second inspection image with the second referenceimage. The first reference image and the second reference image eachinclude pixels smaller than pixels of the first inspection image and thesecond inspection image.

BRIEF DESCRIPTION OF DRAWINGS

A more complete appreciation of the present disclosure and many of theattendant aspects thereof will be readily obtained as the same becomesbetter understood by reference to the following detailed descriptionwhen considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic view illustrating a defect inspecting deviceaccording to an exemplary embodiment of the present disclosure;

FIG. 2 is a flowchart illustrating a method of inspecting a defectaccording to an exemplary embodiment of the present disclosure;

FIG. 3 is a view illustrating an inspection region of a photomaskinspected by a defect inspecting device according to an exemplaryembodiment of the present disclosure;

FIGS. 4A, 4B, 5A and 5B are views illustrating operations of a method ofinspecting a defect according to an exemplary embodiment of the presentdisclosure; and

FIG. 6 is a graph illustrating an effect of an exemplary embodiment ofthe present disclosure.

DETAILED DESCRIPTION

In describing exemplary embodiments of the present disclosureillustrated in the drawings, specific terminology is employed for sakeof clarity. However, the present disclosure is not intended to belimited to the specific terminology so selected, and it is to beunderstood that each specific element includes all technical equivalentswhich operate in a similar manner.

FIG. 1 is a schematic diagram illustrating a defect inspecting deviceaccording to an exemplary embodiment of the present disclosure.

With reference to FIG. 1, the defect inspecting device may include astorage device 205, a reference image generator 210, an inspection imagegenerator 215, an inspection image processor 220, an operation processor230, and a display device 240. The inspection image generator 215 andthe inspection image processor 220 may constitute an image inspector217.

The inspection image generator 215 may include a light source 102, suchas extreme ultraviolet (EUV), deep ultraviolet (DUV), or an i-line lightsource, and may include a light splitter 160 diving light from the lightsource 102 into a transmitted light path 100A and a reflected light path100B. A filter 104 including a filter suitable for a wavelength of lightemitted from the light source 102 and an illumination aperture 106 foradjusting numerical aperture (NA) and coherency of the light emittedfrom the light source 102. Both the illumination aperture 106 and thefilter 104 may be disposed between the light source 102 and the lightsplitter 160.

In addition, ON/OFF shutters 170A and 170B, may be configured to blockor pass light transmitted on the transmitted light path 100A and thereflected light path 100B. The ON/OFF shutters 170A and 170B may bedisposed on the transmitted light path 100A and the reflected light path100B, respectively. Thus, light transmitted on the transmitted lightpath 100A and the reflected light path 100B may be blocked or may travelaccording to an ON/OFF state of the ON/OFF shutters 170A and 170B.Therefore, an image may be selectively acquired using either transmittedlight or reflected light. For example, the transmitted light may be usedto inspect photomasks while the reflected light may be used to inspectintegrated circuits, or vice versa.

A condensing lens 110 is configured to condense light having beenreflected by a reflector 161 and having passed through a lower aperture108 so that the light may be transmitted to a first surface of aphotomask 150. The condensing lens 110 may be disposed on thetransmitted light path 100A. The first surface of the photomask 150 isthe surface on which mask patterns are not disposed. Light transmittedalong the reflected light path 100B may be directed onto a secondsurface of the photomask 150 after being reflected by the reflector 162.The second surface refers to a surface of the photomask 105 on which themask patterns are disposed. The first surface and the second surface areopposite sides of the photomask 105. Reflected light directed to thesecond surface of the photomask 150 along the reflected light path 100Band transmitted light transmitted through the photomask 150 along thetransmitted light path 100A may be transmitted to an optical sensor 130through an objective lens 112, a magnification projection lens 116, andan upper aperture 120. Therefore, transmitted light travels along thetransmitted light path 100A to pass through the photomask 150 and beincident on the optical sensor 160 and reflected light travels along thereflected light path 100B to be reflected from the surface of thephotomask 150 and be incident on the optical sensor 130. The opticalsensor 130 may be provided as a time delay integration (TDI) sensorhaving a plurality of pixels (e.g., 1024×2048 pixels). The photomask 150may be mounted on a table and the optical sensor 130 may scan inspectionregions of the photomask 150, while the table is moved.

The inspection image processor unit 220 may convert light received bythe optical sensor 130 into an electrical signal to form inspectionimages of the inspection regions (including a first inspection image ofa first inspection region, a second inspection image of a secondinspection region, or the like). Image information of the inspectionimages may be transmitted to the operation processor 230.

The reference image generator 210 may read design layout data stored inthe storage device 205 to generate reference images (including a firstreference image, a second reference image, or the like) used in asimulation-based inspection process. Image information of the referenceimages may be transmitted to the operation processor 230. The referenceimages may include pixels smaller than those of the inspection images.

The operation processor 230 may extract a first coordinate offset bycomparing the first inspection image with the first reference image andmay extract a second coordinate offset by comparing the secondinspection image with the second reference image. The operationprocessor 230 may calculate an alignment offset between the firstinspection image and the second inspection image using the firstcoordinate offset and the second coordinate offset. The operationprocessor 230 may align the first inspection image and the secondinspection image to be offset by the alignment offset extracted from thefirst coordinate offset and the second coordinate offset and may thencompare the first inspection image with the second inspection image in adie-to-die manner. The operation processor 230 may extract a gray leveldifference value from gray level data of the first inspection image andof the second inspection image and compare the gray level differencevalue with a predetermined threshold value, thereby determining whethera defect is present in the second inspection region. For example, if thegray level difference value is greater than the predetermined thresholdvalue then it may be determined that a defect is present. The operationprocessor 230 may include at least one microprocessor to compare imagesdescribed above and detect a defect. The operation processor 230 mayfurther include a data storage device to store the images, offset valuesdescribed above, defect inspection results, and the like.

The display device 240 may display images formed by the reference imagegenerator 210 and the inspection image generator 215 or informationobtained in the operation processor 230. An inspection object to beinspected by a defect inspecting device is not limited to a photomask.The inspection object may be an integrated circuit such as a waferhaving circuit patterns formed thereon. In this case, the defectinspecting device may use inspection images by reflected light.

FIG. 2 is a flowchart illustrating a method of inspecting a defectaccording to an exemplary embodiment of the present invention. In thismethod, rather than performing direct die-to-die image comparison,exemplary embodiments of the present invention first perform acomparison of each die image with a reference image so that offsetvalues may be calculated. Then, in performing image comparison, theoffset values are used to ensure the comparison is well aligned. Thisapproach has a distinct advantage over approaches of the related art inthat without the use of offset values, images may be slightly misalignedduring die-to-die image comparison, and this slight misalignment mayalter gray level value comparison and lead to the rejection of good diesand masks as defective. Accordingly, exemplary embodiments of thepresent invention have the advantage of greater manufacturing yield asfewer good dies and masks are rejected as defective.

With reference to FIG. 2, the method of inspecting a defect may comprisegenerating reference images (including a first reference image, a secondreference image, or the like) from design layout data in S1, obtaininginspection images (including a first inspection image, a secondinspection image, or the like) from an inspection object in S2,extracting a first coordinate offset by comparing the first inspectionimage with the first reference image and extracting the secondcoordinate offset by comparing the second inspection image with thesecond reference image in S3, comparing the first inspection image withthe second inspection image in a die-to-die manner using the firstcoordinate offset and the second coordinate offset in S4, anddetermining whether a defect is present in the second inspection imagein S5. In addition, the method of inspecting a defect may furtherinclude determining whether an inspection region remains to be inspectedin S6. In a case in which the inspection region remains to be inspected,operations may be performed again from the obtaining inspection imagesin S2, but this time, using the next inspection region.

The generating of the reference images in S1 may include generating afirst reference die image corresponding to a single die using asimulation process after reading the design layout data and may includegenerating a second reference die image by aligning the same image asthe first reference die image at a predetermined pitch (a pitch betweendies extracted from the design layout data). The first reference imagemay be provided as an image in which the first reference die image isdivided into a predetermined unit (e.g., a patch unit to be subsequentlydescribed). In the same manner as a case described above, the secondreference image may be provided as an image in which the secondreference die image is divided into a predetermined unit (e.g., a patchunit to be subsequently described). The first reference image and thesecond reference image correspond to the same region in the firstreference die image and the second reference die image, respectively,but differ only in terms of coordinates. The first reference image andthe second reference image may include pixels smaller than those of thefirst inspection image and the second inspection image. For example, thefirst reference image and the second reference image may each includepixels of several nanometers or less (e.g., 1 nm), while the firstinspection image and the second inspection image may each include pixelsof tens of nanometers (e.g., 72 nm).

The obtaining inspection images (including the first inspection image,the second inspection image, or the like) in S2 may comprise mounting aphotomask including a first die and a second die disposed adjacent tothe first die in the inspection image generator 215; aligning thephotomask; and directing light onto the photomask (e.g. directing lightonto the first die; and directing light onto the second die).

According to an exemplary embodiment of the present invention, the abovesteps may be automated under the control of the operation processor 230.The operation processor 230 may execute an algorithm in the performanceof the above steps. For example, an algorithm for inspecting defects mayinclude the steps of generating a reference image, obtaining inspectionimages, extracting offset values, extracting gray level differencevalues, determining if the difference values are within the thresholdand determining whether the inspected object is defective, as shown inFIG. 2 and described in detail above.

With reference to FIG. 3, the step of obtaining inspection images(including the first inspection image, the second inspection image, orthe like) will be described in greater detail. A photomask MK mayinclude a plurality of die patterns. A die is an integrated circuit,also referred to as a microchip or simply as a chip. Each of the diepatterns of the photomask may occupy a rectangular region on thephotomask and so the photomask may comprise a plurality of suchrectangular regions. A rectangular region of the photomask, describedabove, may be referred to as a ‘swath’. The dies may include a pluralityof swaths S_1 to S_N. In addition, a plurality of inspection units maybe defined within each swath. The inspection unit may be referred to asa ‘patch’. The swath may include a plurality of patches P_1 to P_M.Respective swaths S_1 to S_N may have a rectangular shape that is longerin an X direction (or a Y direction) and may be aligned substantially inthe Y direction (or the X direction). A size of respective patches P_1to P_M may correspond to a size of a TDI sensor. An inspection image ofrespective patches P_1 to P_M may include a plurality of pixels PX.

An inspection image of the photomask MK may be captured by an inspectionimage generator 215. The inspection image of the photomask (MK) may becaptured in each swath. When an inspection image of each die iscaptured, an operation of a table on which the photomask MK is mountedmay be controlled so that respective swaths S_1 to S_N may becontinuously and successively scanned. While the table is moved in the Xdirection or the Y direction, a first swath S_1, a second swath S_2, . .. and an Nth swath S_N may be sequentially captured. Inspection imagesof all dies of the photomask MK may be captured using the same process.

The inspection image processor 220 may process an inspection image,having been captured, and may transmit image information of theinspection image to the operation processor 230.

The operation processor 230 may be responsible for extracting a firstcoordinate offset by comparing the first inspection image with the firstreference image and extracting a second coordinate offset by comparingthe second inspection image with the second reference image in S3. Theoperation processor 230 may further be responsible for comparing thefirst inspection image with the second inspection image in a die-to-diemanner using the first coordinate offset and the second coordinateoffset in S4. The first inspection image and the second inspection imagemay be provided as images divided into patch units. The images dividedinto the patch units may be referred to as a patch inspection image. Forexample, the first inspection image may be provided as a first patchinspection image obtained from a region of the first die, while thesecond inspection image may be provided as a second patch inspectionimage obtained from a region of the second die. The first referenceimage and the second reference image may also be provided as imagesdivided into patch units among reference images generated by simulation.

With reference to FIGS. 4A and 4B, the extracting of a first coordinateoffset by comparing the first inspection image with the first referenceimage and the extracting of a second coordinate offset by comparing thesecond inspection image with the second reference image in S3 will bedescribed in greater detail below.

With reference FIG. 4A, extracting a first coordinate offset Δ(X1, Y1)by comparing a first inspection image A1 with a first reference image B1may include aligning the first inspection image A1 and the firstreference image B1 to allow the gray level difference value to be aminimum. A distance between a first vertex disposed on a bottom left ofthe first inspection image A1 and a first reference vertex withreference coordinate (X1, Y1) disposed on a bottom left of the firstreference image B1 in the X direction and the Y direction may also bedetermined.

With reference to FIG. 4B, extracting a second coordinate offset Δ(X2,Y2) by comparing a second inspection image A2 with a second referenceimage B2 may include aligning the second inspection image A2 and thesecond reference image B2 to allow the gray level difference value to bea minimum. A distance between a second vertex disposed on a bottom leftof the second inspection image A2 and a second reference vertex withreference coordinate (X2, Y2) disposed on a bottom left of the secondreference image B2 in the X direction and the Y direction may also bedetermined.

The first coordinate offset and the second coordinate offset may bedifferent from each other.

With reference to FIGS. 5A and 5B, the comparing the first inspectionimage with the second inspection image in a die-to-die manner using thefirst coordinate offset and the second coordinate offset in S4 will bedescribed in detail below.

With reference to FIG. 5A, according to an exemplary embodiment of thepresent disclosure, the operation processor 230 may calculate analignment offset between the first inspection image A1 and the secondinspection image A2 using the first coordinate offset Δ(X1, Y1) and thesecond coordinate offset Δ(X2, Y2). The operation processor 230 mayalign the first inspection image A1 and the second inspection image A2to be offset by an alignment offset extracted from the first coordinateoffset Δ(X1, Y1) and the second coordinate offset Δ(X2, Y2). Since thealignment offset may be less than a pixel size of a reference image, analignment error may be reduced to, for example, several nanometers orless.

After the first inspection image and the second inspection image arealigned, the gray level difference value may be extracted from graylevel data of the first inspection image and of the second inspectionimage. According to an exemplary embodiment of the present disclosure,dispersion of an extracted gray level difference value may also bereduced.

With reference to FIG. 5B, in a comparative example, since the firstinspection image and the second inspection image may be directlyaligned, an alignment error similar to a pixel size of an inspectionimage or less (e.g., tens of nanometers or less) may occur.

In a case in which the first inspection image and the second inspectionimage are directly aligned, as in the comparative example, when thepixel size of the inspection image is reduced, the alignment error maybe reduced to some extent. However, a time needed to obtain the imageand an operational time of the inspection image may be increased,thereby degrading throughput of an inspection device. In this way,exemplary embodiments of the present invention have an advantage overthe comparative example in terms of the time needed to perform theinspection.

However, according to an exemplary embodiment of the present disclosure,the alignment error may be significantly improved without degradation ofthe throughput of the inspection device, thereby providing advantages ofgreater efficiency and greater quality.

In the determining whether a defect is present in the second inspectionimage in S5, the gray level difference value may be compared with apredetermined threshold value, thereby determining whether a defect ispresent in the second inspection region.

FIG. 6 is a graph illustrating an effect of an exemplary embodiment ofthe present disclosure.

Results illustrated in FIG. 6 show the same defect using both aninspection method of a comparative example and an inspection method inaccordance with an exemplary embodiment of the present invention. Withreference to FIG. 6, it can be confirmed that, in the case of theexemplary embodiment of the present disclosure, as compared with thecomparative example, a measure of dispersion of a detection signal(e.g., a gray level difference value) is significantly lower. In thecase of the comparative example, 1σ is about 5.0, while, in the case ofthe exemplary embodiment of the present disclosure, 1σ is about 2.0.Dispersion of the detection signal was improved by reducing thealignment error functioning as a factor causing dispersion of thedetection signal. Repeatability and reproducibility of the inspectiondevice was improved.

As set forth above, according to exemplary embodiment of the presentdisclosure, a method of inspecting a defect and a defect inspectingdevice, having improved repeatability and reproducibility, may beprovided.

While exemplary embodiment of the present disclosure have been shown anddescribed above, it will be apparent to those skilled in the art thatmodifications and variations could be made without departing from thescope of the present inventive concept.

What is claimed is:
 1. A defect inspecting, apparatus, comprising: areference image generator configured to generate a first reference imageand a second reference image from design layout data, the design layoutdata including a repeating pattern of a same die spaced apart at apredetermined pitch; an image inspector configured to obtain a firstinspection image of a first inspection region of a photomask and asecond inspection image of a second inspection region of the photomask,the second inspection region being spaced apart from the firstinspection region by the predetermined pitch; and an operation processorconfigured to extract a first coordinate offset by comparing the firstinspection image with the first reference image and to extract a secondcoordinate offset by comparing the second inspection image with thesecond reference image.
 2. The defect inspecting apparatus of claim 1,wherein the first reference image and the second reference image eachinclude pixels smaller than pixels of the first inspection image and thesecond inspection image.
 3. The defect inspecting apparatus of claim 1,wherein the operation processor is further configured to extract a graylevel difference value from gray level data of the first inspectionimage and of the second inspection image and to determine whether adefect is present in the second inspection region by comparing the graylevel difference value with a predetermined threshold value.
 4. Thedefect inspecting apparatus of claim 1, wherein the image acquisitiondevice comprises: a light splitter dividing light emitted from a lightsource into a transmission path and a reflection path; and an opticalsensor configured to receive light having passed through the photomaskand light having been reflected by the photomask.
 5. A defect inspectingapparatus, comprising: a reference image generator configured togenerate a first reference image and a second reference image fromdesign layout data; an image inspector configured to obtain a firstinspection image of a first inspection region of a photomask and asecond inspection image of a second inspection region of the photomask;and an operation processor configured to extract a first coordinateoffset by comparing the first inspection image with the first referenceimage and to extract a second coordinate offset by comparing the secondinspection image with the second reference image, wherein the operationprocessor is further configured to align the first inspection image andthe second inspection image to be offset by an alignment offsetextracted from the first coordinate offset and the second coordinateoffset and then compare the first inspection image with the secondinspection image in a die-to-die manner.
 6. The defect inspectingapparatus of claim 5, wherein the first reference image and the secondreference image each include pixels smaller than pixels of the firstinspection image and the second inspection image.
 7. The defectinspecting apparatus of claim 5, wherein the operation processor isfurther configured to extract a gray level difference value from graylevel data of the first inspection image and of the second inspectionimage and to determine whether a defect is present in the secondinspection region by comparing the gray level difference value with apredetermined threshold value.
 8. The defect inspecting apparatus ofclaim 5, wherein the image acquisition device comprises: a lightsplitter dividing light emitted from a light source into a transmissionpath and a reflection path; and an optical sensor configured to receivelight having passed through the photomask and light having beenreflected by the photomask.